1. Field of the Invention
The present invention relates to molds and processes for forming plastic articles by molding. More particularly, the present invention relates to molds and processes for forming plastic articles on metal substrates.
2. Description of the Related Art                a. Passive Molds        
It is known to form thermoplastics and other materials by injection into passive molds. It is understood that passive molds comprise a cavity in metal or other material into which a semi-liquid substance is injected or extruded, usually under light pressure, until the mold cavity is filled. The injected or extruded work piece is allowed to set up and then the mold is opened and the work piece (often comprising polymers) is removed and allowed to cure prior to storage or shipping or additional forming (for example, by machining) at subsequent stations. There can be multiple cavities within the same mold and the material can be distributed via a manifold.                b. Molding Plastic on Metal        
There are a number of potential applications for composites comprising plastic members formed along the edges of substrates such as metal strips and other materials. Examples include plastic couplings formed along the ends of metal pipes.
The inherent difficulty of forming a plastic component to a specified shape along the edge of a metal strip, with adequate bonding between the two materials, is increased by the necessity of using a slotted mold so that only the edge of the strip is within the mold. The difficulties may be increased even further (a) if it is necessary to move the metal substrate through the mold to permit forming an elongated plastic component or to ensure coverage of the metal by the plastic; (b) when forming the plastic on a curved substrate; and (c) for applications which require that the plastic-metal composite provide a seal against fluid leakage or against corrosion of the metal. Most or all of these considerations are involved in any attempt to form plastic couplings on pipe ends.